This invention relates to centrifugal separators used for the separation of the liquid and gaseous phases of a mixture thereof.
Many processes require the separation of a liquid from its own vapor or from a gas of nearly the same molecular weight. For example, a geothermal well may produce a mixture of steam and a highly saline geothermal brine at a high temperature and pressure and at a relatively high rate of flow. In order to obtain clean, dry steam for power generation and to enable recovery of the minerals dissolved in the brine, the two phases of the produced mixture must be separated from each other.
Although various types of separators have been designed, those chiefly used for this purpose achieve their results through gravitational separation of the two phases. Gravitational separation can occur if the mixture is confined under nearly static flow conditions for a sufficient time to allow the liquid phase to settle out of the mixture. Separators of this type are not useful, however, in the handling of high rates of flow because of the extremely large vessels that would be required to provide the necessary residence time for such separation.
In order to handle high rates of flow, centrifugal, or cyclone, separators are used to cause the fluid mixture to move in a helical path so that gravitational separation is accomplished by outward migration of the liquid particles from the axis of the helix. Typically, such separators comprise a vertical cylinder in which the fluid is introduced tangentially at the lower end of the cylinder, with the gaseous phase being removed from the upper end of the cylinder. The curvature of the cylinder wall forces the incoming fluid into a vortex field with a high centrifugal acceleration, and the gas and entrained liquid will then move helically up the cylinder towards the gaseous phase outlet, with the entrained liquid gravitating outwardly to the cylinder wall to collect thereon during such fluid movement. The gas, with whatever liquid portions that have not been separated therefrom, will then exit the separator from the upper outlet.
The separated water in the separator will also be urged to flow helically upwardly therein, due to the tangential and vertical components of velocity resulting from the centrifugal acceleration and vertical movement of the fluid through the separation. Although centrifugal separators have been designed to remove the upwardly flowing collected liquid from a liquid outlet above the fluid inlet, e.g. U.S. Pat. No. 3,488,924, the typical commercially used separator utilizes a bottom outlet, which will allow removal of the collected liquid after the liquid has accumulated on the separator wall to the extent that its mass will cause downward drainage of the separated liquid.
Ideally, a separator should provide a complete separation between the two phases and cause no loss of pressure in the fluid flowing through the system. Such ideal, of course, cannot be achieved in the design of a centrifugal separator. The separator must allow sufficient residence time to provide a long enough helical flow path of the fluid for the liquid phase to separate from the gaseous phase. An increase in the length of the flow path will increase, proportionately, the pressure drop of the fluid through the flow path. The efficiency of separation can be increased by increasing the centrifugal acceleration of the fluid. However, this will likewise increase the velocity of the fluid as it moves through the flow path, and the pressure drop will increase proportionately to the square of this velocity.
Thus, in the design of a conventional centrifugal separator, the diameter of the separator represents a compromise between conflicting considerations. The smaller diameter, the greater will be the centrifugal acceleration and separation efficiency, but also the pressure drop through the system will increase. The pressure drop can be reduced by increasing the diameter, but, by so doing, the separation efficiency will decrease.
Another problem encountered in the operation of centrifugal separators, and particularly involving the separation of a liquid from its own vapor, is that of reentrainment of the separated liquid back into the vapor. The liquid phase, of course, shows a tendency to adhere to the separator wall in preference to the accompanying vapor phase. If the relative velocity between the separated liquid phase and vapor phase at the interface thereof is sufficiently low, there will be little, if any, reentrainment of the separated liquid back into the vapor phase. However, if such relative velocity increases, then the degree of reentrainment will also increase. Since the separated liquid and vapor outlets must be spaced physically apart there will be a difference in the directions of flow of the separated liquid and vapor at their interface which will cause unwanted reentrainment thereat.
Another aspect of the reentrainment problem is that the separated liquid will have a high component of vertical velocity imparted thereto which will cause the liquid to climb completely up the separator wall. Typically, the vapor outlet will be formed by a tubing which extends through the dome or wall of the separator and has an opening inside the separator from which the vapor can exit. Some of the climbing liquid will also flow along such tubing to the opening thereof and be aspirated back into the exiting vapor. If the vapor outlet is not located centrally in the dome, the climbing liquid will accumulate thereat until it has sufficient mass to drop back down into the swirling vapor phase and be reentrained therein.
A still further problem of conventional centrifugal separators is encountered in a steam-brine separator process when the liquid phase has a high concentration of total dissolved solids. Any surface of the separator that is wetted by the separated liquid must be kept sufficiently wetted so that the solids will remain in solution, not scale out in the separator wall, nor be dehydrated by exposure to the vapor.